Steam valve

ABSTRACT

A steam valve is provided with a valve body, a valve seat, and a valve rod provided for the valve body. The valve body has a bottom portion to which a recessed portion is formed, and the recessed portion has an edge, which is positioned on an upstream side of a place at which a shock wave is generated in steam flowing through a steam path defined by the valve body and the valve seat.

CROSS REFERENCE TO RELATED APPLICATION

This application is based upon and claims the benefit of priority fromprior Japanese Patent Application No. 2004-250408 filed on Aug. 30,2004, the entire contents of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to steam valves, for example, forpower-generating facilities, and specifically, to a steam valve that isprovided with an improved valve body to ensure and maintain a steadyflow of steam passing through a gap between the valve body and a valveseat.

2. Related Art

Generally, steam turbines used in power-generating facilities, such asthermal power plants and nuclear power plants, are provided with manysteam valves for controlling the amount of steam flow according to loadchanges, or for cutting off the supply of steam in response to anemergency.

A steam control valve, among steam valves used in thermal power plantsand the like, deals with a large amount of hot and high-pressure steamflow and, therefore, frequently opens and closes its valve body. Thiscauses steam to drift or swirl in the beginning process of opening thevalve body or in the process of closing the valve body. Such steamturbulence results in noise, vibration, erosion, and accidents, such ascracks in a connecting part of a valve rod supporting the valve body.

A number of inventions have been provided to prevent such problems andaccidents, such as disclosed in, for example, documents or publicationsof Japanese Unexamined Patent Publications Nos. SHO 56-109955, HEI9-72430, HEI 9-210244, HEI 10-89494, HEI 10-299909, and HEI 10-299910.

In particular, Japanese Unexamined Patent Publication No. SHO 56-109955discloses a so-called pioneering technique that was developed when noiseand vibration were serious issues.

In recent power-generating facilities planning to introduceultra-supercritical pressure technology, steam conditions (temperatureand pressure) increase as the single capacity of a steam turbineincreases. Such increases in steam conditions cause noise and vibrationto be a major concern again.

To reduce noise and vibration, techniques that are the results ofdevelopment and are disclosed in the above-described documents have beenconventionally used in dealing with subcritical pressure at a pressureof 169 ata and a temperature of 538° C., and supercritical pressure at apressure of 246 ata and a temperature of 538° C. However, initiatives tointroduce ultra-supercritical pressure technology require new techniquesto be developed and cause the above-described conventional techniques toreach their limits.

In particular, a common challenge for turbine producers to furtherreduce noise and vibration is to determine the size of the edge of thevalve body such that a steady steam flow can be ensured and maintainedeven if the above-described steam conditions increase further.

SUMMARY OF THE INVENTION

The present invention was conceived in consideration of the abovecircumstances, and an object of the present invention is to provide asteam valve having an improved structure capable of ensuring a andmaintaining a stable steam flow around a valve body at a time of openingor closing the valve body.

The above and other objects can be achieved according to the presentinvention by providing, in one aspect, a steam valve comprising:

a valve body;

a valve seat; and

a valve rod provided for the valve body, the valve body having a bottomportion to which a recessed portion is formed, the recessed portionhaving an edge, wherein the edge is positioned on an upstream side of aplace at which a shock wave is generated in a steam flowing through asteam path defined by the valve body and the valve seat.

In this aspect, the edge is positioned at a critical point of the steamflowing through the steam path, which is defined by the valve body andthe valve seat.

In preferred examples of the above aspects, a range of an edge diameterDi of the edge may be set toDi≧0.90Doand a range of a valve-seat internal diameter Dth of the valve seat isset toDth≧0.80Dowhere Do is a valve seat diameter of the valve seat,

The range of an edge diameter Di of the edge may be set toDi=(0.90 to 1.0)Doand a range of a valve-seat internal diameter Dth of the valve seat maybe set toDth=(0.80 to 0.90)Dowhere Do is a valve seat diameter of the valve seat.

A depth h of the recessed portion may be set to h≦15 mm and a tilt angleθ of the edge is set to θ=45°.

The range of a curvature radius R of the valve body may be set toR=(0.52 to 0.6)Doand a range of a curvature radius r of the valve seat may set tor≧0.6Dowhere Do is a valve seat diameter of the valve seat.

The range of a curvature radius R of the valve body may be set toR=(0.6 to 0.85)Doand the range of a curvature radius r of the valve seat may be set tor=(0.45 to 0.6)Dowhere Do is a valve seat diameter of the valve seat.

The range of a curvature radius R of the valve body and a curvatureradius r of the valve seat may be set toR=r=(0.45 to 0.85)Dowhere Do is a valve seat diameter of the valve seat.

The range of a maximum valve lift La of the valve body combined with thevalve seat may be set toLa=(0.25 to 0.35)Dowhere Do is a valve seat diameter of the valve seat.

The range of a maximum valve lift Lb of the valve body separated fromthe valve seat may be set toLb≦0.25Dowhere Do is a valve seat diameter of the valve seat.

In addition, the valve body, the valve seat, and the recessed portionare formed with either one of a hardened heat-treated portion and ahard-alloy-surfaced portion.

In a further aspect of the present invention, there is also provided asteam valve comprising:

a valve body;

a valve seat; and

a valve rod provided for the valve body,

the valve body having a bottom portion to which a recessed portion isformed, the recessed portion having an edge, wherein a groove isprovided along an entire circumferential surface of the valve body.

In this aspect, the groove is formed on a downstream side of a criticalpoint of steam flowing through a steam path defined by the valve bodyand the valve seat.

In a still further aspect, there is also provided a steam valvecomprising:

a valve body;

a valve seat; and

a valve rod provided for the valve body,

the valve body having a bottom portion to which a recessed portion isformed, the recessed portion having an edge, wherein at least one of thevalve body and the valve seat is provided with a knurled portion formedon a downstream side of a critical point of steam flowing through asteam path defined by the valve body and the valve seat.

In this aspect, the knurled portion may be provided along an entirecircumferential surface of the valve body or valve seat.

According to the steam valve of the present invention having theimproved structure mentioned above, noise, vibration and the like of thesteam flow passing the steam path defined by the valve body and thevalve seat can be preferably prevented from causing and the stable steamflow around the valve body can be ensured and maintained.

The nature and further characteristic features of the present inventionwill be made more clear from the following descriptions made withreference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings:

FIG. 1 is a schematic diagram of a steam valve according to a firstembodiment of the present invention;

FIG. 2 is a pressure distribution diagram showing the pressuredistribution of steam that flows through a steam path defined by a valvebody and a valve seat;

FIG. 3 is a partial enlarged view of FIG. 2;

FIG. 4 is a schematic diagram of a steam valve according to a secondembodiment of the present invention;

FIG. 5 is a schematic diagram of a first combined-type steam valve inwhich a valve body is connected to a valve rod via a pin, i.e., pinconnection;

FIG. 6 is a schematic diagram of a second combined-type steam valve inwhich a valve body and a valve rod are cut out of the same piece of rawmaterial;

FIG. 7 is a schematic diagram of a separate-type steam valve in whichthe valve body and the valve rod are separated;

FIG. 8 is a schematic diagram of a steam valve according to a thirdembodiment of the present invention;

FIG. 9 is a schematic diagram of a steam valve according to a fourthembodiment of the present invention;

FIG. 10 is a schematic diagram of a steam valve according to a fifthembodiment of the present invention; and

FIG. 11 is a schematic diagram of a thermal power plant.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Steam valves according to embodiments of the present invention will bedescribed hereunder with reference to the drawings and referencenumerals added thereon.

Before the description, a power-generating facility to which a steamvalve of the present invention is applicable, and the behavior of steamflow during the opening and closing process in a steam valve of thepresent invention will be described for better understanding of thesteam valve of the present invention.

FIG. 11 is a schematic diagram of a power-generating facility, e.g., apower plant, in which a steam valve of the present invention is used.

This power-generating facility includes a boiler 1, a high-pressureturbine 2, an intermediate-pressure turbine 3, a low-pressure turbine 4,and a condensate/feedwater system 5. The power-generating facilityallows steam generated in the boiler 1 to pass through a main steam-stopvalve (main stop valve) 6 and a steam governing valve (steam controlvalve) 7, causes the high-pressure turbine 2 to perform expansion work,and supplies exhaust steam from the high-pressure turbine 2 to areheater 10 of the boiler 1 via a check valve 9 of a low temperaturereheat pipe 8.

The reheater 10 reheats the exhaust steam supplied from thehigh-pressure turbine 2 via the low temperature reheat pipe 8. Thereheater 10 heats the exhaust steam into reheat steam ofultra-supercritical pressure, supplies the reheat steam, via areheat-steam stop valve 11 and an intercept valve 12, to theintermediate-pressure turbine 3 that performs expansion work. The steamexhausted from the intermediate pressure turbine 3 is further suppliedto the low-pressure turbine 4 that also performs expansion work. Thesteam exhausted from the low pressure turbine 4 is introduced to acondenser 13 of the condensate/feedwater system 5. The condenser 13condensates the exhaust steam into condensate.

The condensate condensed in the condenser 13 is pressurized by afeedwater pump 14 of the condensate/feedwater system 5 and sent back asfeedwater to the boiler 1.

This power plant includes a high-pressure-turbine bypass pipe 16branching off from the exit of the boiler 1 and connected via ahigh-pressure-turbine bypass valve 15 to the low temperature reheat pipe8, and also includes a low-pressure-turbine bypass pipe 18 branching offfrom the exit of the reheater 10 and connected via alow-pressure-turbine bypass valve 17 to the condenser 13. Thisconfiguration enables the boiler 1 to operate independently during thestart-up operation of the high-pressure turbine 2, intermediate-pressureturbine 3, and low-pressure turbine 4, thereby enhancing the operabilityof the plant.

Three configurations of typical steam valves are shown in FIGS. 5 to 7.The steam valves 7, such as the main stop valve 6 and the steam controlvalve 7, can be selected and applied to from these types. The threetypes are as follows: a first combined type (a valve body is connectedto a valve rod via a pin) shown in FIG. 5; a second combined type (avalve body and a valve rod are cut out of the same piece of rawmaterial) shown in FIG. 6; and a separate type (a valve body and a valverod are separated (double valve type)) shown in FIG. 7.

FIG. 5 shows the first combined-type steam valve in which a valve chest20 housed in a valve casing 19 is provided with a valve seat 21 on whicha spherical valve body 22 that can move into and out of contact with thevalve seat 21 is placed.

The spherical valve body 22 is connected via a pin 23 to a valve rod 24,which is further connected via a lever 26 to an oil cylinder 25.Supplying and draining hydraulic fluid to and from the oil cylinder 25drives the lever 26 to be swung in the direction indicated by an arrowAR with a supporting point 27 as the fulcrum. The driving forcetransmitted to the valve rod 24 causes the valve body 22 to move intoand out of contact with the valve seat 21 (open and close with respectto the valve seat 21), thereby controlling the amount of steam passingthrough the valve chest 20.

FIG. 6 shows the second combined-type steam valve that includes thevalve body 22 and valve rod 24 cut out of the same piece of material.Sliding the valve rod 24 against a bushing 30 mounted inside a stand 29allows the valve body 22 to move into and out of contact with the valveseat 21 (open and close with respect to the valve seat 21).

FIG. 7 shows the separate-type (double valve type) steam valve in whichthe valve rod 24 and auxiliary valve body 31 cut out of the same pieceof raw material are housed in a main valve body 32. Uponoperation-start, the steam valve causes the valve rod 24 to slideagainst the bushing 30 to open the auxiliary valve body 31, therebyallowing steam to be supplied from a gap between a balance chamber 33and the main valve body 32. Then, the steam valve brings a shoulder 38of the auxiliary valve body 31 into contact with the main valve body 32,allows the main valve body 32 to slide within the balance chamber 33,and causes the main valve body 32 to move into and out of contact withthe valve seat 21 (open and close with respect to the valve seat 21).

To achieve a higher level of ultra-supercritical pressure, the presentinventors modeled a steam valve having the characteristics of one of theabove-described types and carried out experiments over and over again,based on data obtained from Japanese Unexamined Patent Publication No.SHO 56-109955.

Observations and measurements of the behavior and pressure distributionof steam flowing through a steam path defined by the valve seat 21 andthe valve body 22 (main valve body 32) showed that unsteady shock wavesaccompanied by large pressure fluctuations causing vibration to thevalve body 22 and the like were generated in steam flowing through thesteam path.

Since a steady flow of steam can be ensured by preventing such unsteadyshock waves, the present inventors changed the position and size of anedge of a recessed part, e.g., a depression or a hollow, in the valvebody 22 (including the main valve body 32) over and over again, througha trial and error process, to develop measures to prevent unsteady shockwaves.

Based on the experiments and analysis outlined above, the behavior andpressure distribution of the steam flowing through a steam path betweenthe valve seat 21 and the valve body 22 will be described hereunder indetail with reference to FIG. 1 and FIG. 2.

FIG. 1 is a schematic diagram showing the position and degree of a gapbetween a valve body and a valve seat at a given point at an optionaltime during the process of valve opening or closing in a steam valveaccording to the first embodiment of the present invention.

The steam valve includes a valve seat 34 and a spherical valve body 35(including main valve body). The valve body 35 is provided with arecessed part (e.g., a depression or a hollow) 36 cut and inwardlyrecessed from the bottom. An edge 37 of the recessed part 36 cuts offsteam flowing along the spherical surface of the valve body 35.

A major axis line B connects the center O₁ of the curvature radius R ofthe surface of the valve body 35 with the center O₂ of the curvatureradius r of the surface of the valve seat 34. A line A is a lineparallel with the major axis line B and passes through a position A₁ onthe surface of the valve seat 34 indicating the starting point of asteam path 28. A line C is a line parallel with the major axis line 3and passes through a position C₁ on the edge 37 of the recessed part 36in the valve body 35. The line C passing through the position C₁ has adistance X from the line A passing through the position A₁. A circle onthe major axis line B indicates the minimum path area Ath of the steampath 28 at a certain position and degree of a gap between the valve body35 and the valve seat 34 at a given point at an optional time.

The recessed part 36 at the bottom of the valve body 35 has a depth(height) h. The edge 37 tilts toward the inside of the valve body 35 ata tilt (tilting) angle θ with respect to an axis line L in thelongitudinal direction of the valve body 35 passing through the positionC₁ of the edge 37.

The edge 37 has a diameter Di. The valve seat 34 coming into contactwith the valve body 35 at valve closing has a valve seat diameter Do.The valve seat 34 has a valve-seat internal diameter Dth touching thecurvature radius r.

Entrance-side pressure at the front (upstream side) of the valve body 35is indicated by a character P₁, while exit-side pressure at the rear(downstream side) of the valve body 35 is indicated by a character P₂.The lift (moving distance) of the valve body 35 at a given point at anoptional time is indicated by La (Lb).

FIG. 2 is a pressure distribution diagram of an isentropic flow underideal conditions, which analogize the steam path 28 at a given point atan optional time shown in FIG. 1 with a de Laval nozzle. The verticalaxis represents the ratio of the exit-side pressure P₂ of the valve body35 to the entrance-side pressure P₁ of the valve body 35 (P₂/P₁), whilethe horizontal axis represents the distance X shown in FIG. 1.

Identifiers A₂, B₂, and C₂ on the horizontal axis (distance line X)correspond to positions A₂, B₂, and C₂, respectively, shown in FIG. 1.

Assuming that the exit-side pressure P₂ is lowered with theentrance-side pressure P₁, which is kept constant, the pressuredistribution follows a curve starting at a point a ((P₂/P₁)=1.0) andpassing through a point j and a point k, as indicated by a dashed linein FIG. 2. A steam flow having pressure distribution that follows acurve passing through the point a, the point j, and the point k is asubcritical flow (subsonic flow).

When the exit-side pressure P₂ is lowered further, the pressuredistribution follows a curve indicated by a solid line, and a criticalpoint b is reached in the minimum path area Ath of the steam path 28 onthe major axis line B in FIG. 1. A steam flow having pressuredistribution that follows a curve from the point a to the critical pointb is also a subcritical flow (subsonic flow).

When the exit-side pressure P₂ is lowered further, the pressuredistribution follows a curve starting at the critical point b andpassing through a point C₃, a point d, and a point e. A steam flowhaving pressure distribution that follows a curve from the criticalpoint b to the point e is a supercritical flow (supersonic flow).

When a steam flow is a supercritical flow, a shock wave is generated,for example, at the point C₃ and at the point d, in a directionperpendicular to the steam flow, and the pressure rises to a point f andto a point h, respectively (recovery of static pressure). The pressuredistribution follows a Rankine-Hugoniot (R-H) line starting at thecritical point b and passing through the point f, a point g (maximumpressure), the point h, and a point i, indicated by a chain line.

The steam flow exhibiting a pressure rise to the point f and point h onthe R-H line exhibits a further pressure rise from the point f to apoint m, and from the point h to a point o, producing subcritical flows(subsonic flows).

The pressure distribution diagram in FIG. 2 shows an ideal pressuredistribution of an isentropic flow.

In practice, however, observations showed that, for example, shock wavesgenerated at the point C₃ and point d caused pressure to exceed thepoint f and point h on the R-H line and to reach a point l and point n,respectively.

The steam flow exhibiting a pressure rise to the point 1 and point n dueto the shock waves is in an unstable state and cannot independentlymaintain its steady state. Therefore, as shown in FIG. 3, the point land the point n are shifted to a point l, and a point n, respectively,on an upper stream side of the R-H line. The steady state of the steamflow can thus be maintained. Then, the steam flow exhibiting a pressurerise to the point l and point n due to shock waves becomes artificiallya subcritical flow that follows a curve starting at the point l₁ and thepoint n₁ on the R-H line toward a point m₁ and a point o₁, respectively.

Thus, in an area Δ₁ where a shift from the point l to the point l₁occurs, and in an area Δ₂ where a shift from the point n to the point n₁occurs, pressure fluctuations cause noise and vibration in a steam flow.Moreover, a shift caused by a shock wave continuously occurs toward theupstream side of the R-H line up to the critical point b.

That is, the present inventors found from the observations that a shockwave was generated many times in a range from the critical point b tothe point e, every time when the shock wave caused pressure to risebeyond the R-H line and then shift toward the upstream side of the R-Hline, and such a shift occurred sequentially up to the critical point b.The critical point b corresponds to the minimum path area Ath in thesteam path 28 shown in FIG. 1.

Based on the data and knowledge (information) described above, thepresent inventors focused their attention on the fact that a steam flowfrom the entrance side to the minimum path area Ath in the steam path 28between the valve body 35 and the valve seat 34 was a subcritical flow(subsonic flow), and that no shock wave was generated in that range. Thepresent inventors thus reached the completion of the embodiment of thepresent invention described below.

In the present embodiment, the edge 37 of the recessed part 36 providedon the bottom of the valve body 35 is positioned on the upstream side ofthe steam path 28 and immediately before the place where a shock wave isgenerated in the steam path 28.

In the steam valve, typically noise and vibration are generated incertain range of the valve lift. Therefore, the position of the edge 37might be preferably determined based on the place where a shock wave isgenerated when the valve lift is in the range that generates noise orvibration. Experiments or computer simulations may be determine thevalve lift (and the place where the shock wave is generated) thatgenerates noise or vibration in its steam path 28.

The dimensions of the valve body 35 and the valve seat 34 are set toDi≧0.90DoDth>0.80Dowhere Di is the diameter of the edge 37, Dth is a valve-seat internaldiameter touching the curvature radius r of the valve seat 34, and Do isthe valve seat diameter of the valve seat 34 coming into contact withthe valve body 35 at the valve closing time.

While the values described above are within an applicable rangeconfirmed by experiments, it is more preferable that the edge diameterDi and the valve-seat internal diameter Dth be set toDi=(0.90 to 1.0)DoDth=(0.80 to 0.90)Do.

If the edge diameter Di and the valve-seat internal diameter Dth are setto be within the range described above, the minimum path area Ath of thesteam path 28 can be set to have any shape different from that of thegeometric minimum path area formed on the major axis line B connectingthe center C₁ of the curvature radius R of the valve body 35 with thecenter C₂ of the curvature radius r of the valve seat 34.

In this case, the curvature radius R of the valve body 35 and thecurvature radius r of the valve seat 34 are set to one ofR=(0.52 to 0.6)Do,r≧0.6Do;R=(0.6 to 0.85)Do,r=(0.45 to 0.6)Do; andR=r=(0.45 to 0.85)Do,where Do is the seat diameter of the valve seat 34.

The edge diameter Di and the valve-seat internal diameter Dth are bothappropriate values ascertained by experiments.

The tilt angle θ of a surface tilted from the edge 37 of the recessedpart 36 at the bottom of the valve body 35 toward the inside of thevalve body 35 is set to θ=45° with respect to the axis line L thatpasses through the edge 37 and serves as a reference line for the edgediameter Di.

The depth (height) h of the recessed part 36 is set to h≦15 mmregardless of the valve seat diameter Do at the contact point betweenthe valve body 35 and the valve seat 34.

In the steam valve of the present embodiment, considerations need to begiven to noise and vibration during the process of opening the valvebody, as well as to those associated with shock waves described above.

In a known steam valve, a pressure difference between the entrance sideand exit side of a valve body decreases as the valve body moves towardthe full open position. Then, drift and turbulent steam flowing throughthe steam path 28 directly cause the valve body 35 to float toward thefull open position. This occurs repeatedly and causes noise andvibration during the opening process. Such a problem is more serious inthe separate-type steam valve shown in FIG. 7 than in the combined-typesteam valves shown in FIG. 5 and FIG. 6.

The present embodiment takes such circumstances into consideration, andplaces a limit on the lift (stroke) of the valve body 35 so that thrustis always added to the valve-plug entrance-side pressure Pi even if thedifference between the valve-plug entrance-side pressure P₁ and thevalve-plug exit-side pressure P₂ decreases as the valve body 35approaches the full open position.

In the present embodiment, the range of a maximum lift (stroke) La whenthe valve body 35 of each of the combined-type steam valves shown inFIG. 5 and FIG. 6 is at the full-opening position is set toLa=(0.25 to 0.35)Do,while the range of a maximum lift (stroke) Lb when the valve body 35 ofthe separate-type steam valve shown in FIG. 7 is at the full-openingposition is set toLb≦0.25Dowhere Do is a valve seat diameter.

The above-described range of the maximum lift La for the combined-typesteam valves and the maximum lift Lb for the separate-type steam valveare both the applicable ranges ascertained by experiments.

In the steam valve of the present embodiment, the edge 37 of therecessed part 36 provided on the bottom of the valve body 35 ispositioned on the upstream side of and immediately before the placewhere a shock wave is generated in the steam path 28. Moreover, the edgediameter Di and the valve-seat internal diameter Dth are set to bewithin a desirable numerical range ascertained by experiments, while thedimensions of the curvature radius R of the valve body 35 and thecurvature radius r of the valve seat 34 are also set to be appropriatevalues ascertained by experiments. Thus, noise and vibration of steamflow passing through the steam path 28 can be prevented, and steadysteam flow around the valve body 35 can be reliably ensured andmaintained.

Moreover, in the steam valve of the present embodiment, the tilt angle θof a surface tilted from the edge 37 of the recessed part 36 at thebottom of the valve body 35 toward the inside of the valve body 35 isset to θ=45° with respect to the axis line L that passes through theedge 37 and serves as a reference line for the edge diameter Di. Thisprevents steam from entering the recessed part 36 via the edge 37 andthus can prevent erosion caused by stagnant steam in the recessed part36.

Moreover, in the steam valve of the present embodiment, the range of thedepth h of the recessed part 36 provided at the bottom of the sphericalvalve body 35 is set to h≦15 mm such that excitation force generated byfrequent jet flow from the steam path 28 to the recessed part 36 can beweakened. The occurrence of cracks and the like in the edge 37 can thusbe prevented.

Moreover, in the steam valve of the present embodiment, different rangesof valve lift are set for the above-described combined types andseparate type such that noise and vibration generated in the process offully opening the valve body can be reduced. Thus, steady steam flowaround the valve body 35 can be reliably ensured and maintained.

FIG. 4 is a schematic diagram of a steam valve according to the secondembodiment of the present invention. In the drawing, components that areidentical to those in the first embodiment are given the same referencenumerals.

In the steam valve of the present embodiment, the edge 37 of therecessed part 36 provided on the bottom of the valve body 35 ispositioned at the critical point of steam, that is, in the minimum patharea Ath (major axis line B (C) connecting the center of the curvatureradius R of the valve body 35 with the center of the curvature radius rof the valve seat 34) of the steam path 28 at a valve lift positionwhere noise and vibration start to be generated within the range ofvalve lift. The positioning of the edge 37 is based on the valve liftposition where noise and vibration start to be generated because theminimum path area Ath of the steam path 28 changes (varies) depending onthe valve lift position. This embodiment might be achieved by formingthe edge 37 to contact with the valve seat 34 when the valve body 35 iscompletely closed with respect to the valve seat 34.

Further, since the valve lift tends to almost be fixed during the ratedoperation of the steam turbine, the position of the edge 37 may be basedon the valve lift under the rated operation. This might be able toreduce noise and vibration of the steam valve when noise or vibration isrelatively large during the rated operation. As described above, in thesteam valve of the present embodiment, the edge 37 of the recessed part36 provided on the bottom of the valve body 35 is positioned, as is alsoshown in FIG. 2, at the critical point b where no shock wave isgenerated. Thus, noise and vibration of steam flow passing through thesteam path 28 can be prevented, and steady steam flow around the valvebody 35 can be reliably ensured and maintained.

In the steam valve, typically, noise and vibration are generated incertain range of the valve lift. Therefore, the position of the edge 37might be preferably determined based on the critical point b (minimumpath area Ath of the steam path 28) of certain valve lift that generatesnoise or vibration in its steam path 28. Experiments or computersimulations may determine the valve lift, which generates noise orvibration in its steam path 28.

FIG. 8 is a schematic diagram of a steam valve according to the thirdembodiment of the present invention. In the drawing, components that areidentical to those in the first embodiment are given the same referencenumerals.

The steam valve of the present embodiment is provided with at least oneannular groove 39 having semicircular cross-section with a radius ofpreferably 1 mm or less around the valve body 35 in the circumferentialdirection. The annular groove 39 is preferably located on the downstreamside of the major axis line B (minimum path area Ath of the steam path28) at the condition that starts to generate the supercritical(supersonic) stream flow in the steam path 28. As described above, sincethe major axis line B connects the center of the curvature of the valvebody 35 and the center of the curvature of the valve seat 34, theposition of the major axis line B corresponds to the position of minimumpath area Ath of the steam path 28 when the supercritical (supersonic)stream flow starts to be generated.

The present embodiment is based on the fact, as shown in FIG. 2 and FIG.3, that a shock wave generated on the downstream side of the criticalpoint b causes pressure to rise above the R-H line to reach the points land n, which is then shifted to the upstream side of the R-H line. Thegroove 39 changes the surface area of the spherical valve body 35,limits the pressure rise associated with shock waves, allows staticpressure to recover, and suppresses the shock waves, thereby preventingthe pressure rise associated with shock waves from shifting to theupstream side of the R-H line.

As described above, in the present embodiment, the annular groove 39along the circumferential direction of the valve body 35 is positionedon the downstream side of the major axis line B passing through thecenter C₁ of the curvature of the valve body 35 and the center C₂ of thecurvature of the valve seat 34. Since this changes the surface area ofthe valve body 35 and compensates for the pressure rise associated withshock waves, noise and vibration of steam flow passing through the steampath 28 can be prevented, and steady steam flow around the valve body 35can be reliably ensured and maintained.

In the present embodiment, the annular groove 39 is provided along thecircumferential direction of the valve body 35 to change the surfacearea. As shown in FIG. 9, in the fourth embodiment, at least one of thevalve body 35 and the valve seat 34 may be provided with a knurledportion to change the surface area. That is, at least one of a knurledportion (a ridge having an edge) 40 a and a knurled portion (a ridgehaving an edge) 40 b may be provided along the entire circumference ofone of the valve body 35 and the valve seat 34.

Preferably, in this case, the roughness R₁ of the knurled portion 40 aof the valve body 35 is 0.8 S, while the roughness R₂ of the knurledportion 40 b of the valve seat 34 ranges from 1.6 S to 50 S.

Similarly to the third embodiment, the knurled portion 40 a and theknurled portion 40 b of the valve body 35 and the valve seat 34,respectively, are both positioned on the downstream side of the majoraxis line 3 passing through the center of the valve body 35 and thecenter of the valve seat 34.

FIG. 10 is a schematic diagram of a steam valve according to the fifthembodiment of the present invention. In the drawing, components that areidentical to those in the first embodiment are given the same referencenumerals.

To prevent the valve body 35, the recessed part 36, the edge 37, and thevalve seat 34 from being damaged due to oxide scales (iron powder) insteam passing through the steam path 28 between the valve body 35 andthe valve seat 34, the steam valve of the present embodiment is providedwith a hardened heat-treated portion 41 formed by nitriding treatment ora hard-alloy-surfaced portion 42 treated with Stellite (trade name)containing cobalt.

In the thermal power plant in which the ultra-supercritical pressuretechnology is introduced, an increase in the use of austenitic materialcauses a larger amount of oxide scale to be generated. Therefore, theapplication of hardening means, such as those described above, isextremely effective in preventing the valve body 35, the recessed part36, the edge 37, and the valve seat 34 from being damaged.

1. A steam valve comprising: a valve body; a valve seat; and a valve rodprovided for the valve body, the valve body having a bottom portion towhich a recessed portion is formed, the recessed portion having an edge,wherein the edge is positioned on an upstream side of a place at which ashock wave is generated in a steam flowing through a steam path definedby the valve body and the valve seat.
 2. The steam valve according toclaim 1, wherein the edge is positioned at a critical point of the steamflowing through the steam path.
 3. The steam valve according to claim 1,wherein a range of an edge diameter Di of the edge is set toDi≧0.90Do and a range of a valve-seat internal diameter Dth of the valveseat is set toDth≧0.80Do where Do is a valve seat diameter of the valve seat.
 4. Thesteam valve according to claim 1, wherein a range of an edge diameter Diof the edge is set toDi=(0.90 to 1.0)Do and a range of a valve-seat internal diameter Dth ofthe valve seat is set toDth=(0.80 to 0.90)Do where Do is a valve seat diameter of the valveseat.
 5. The steam valve according to claim 1, wherein a depth h of therecessed portion is set to h≦15 mm and a tilt angle θ of the edge is setto θ=45°.
 6. The steam valve according to claim 1, wherein a range of acurvature radius R of the valve body is set toR=(0.52 to 0.6)Do and a range of a curvature radius r of the valve seatis set tor>0.6Do where Do is a valve seat diameter of the valve seat.
 7. Thesteam valve according to claim 1, wherein a range of a curvature radiusR of the valve body is set toR=(0.6 to 0.85)Do and the range of a curvature radius r of the valveseat is set tor=(0.45 to 0.6)Do where Do is a valve seat diameter of the valve seat.8. The steam valve according to claim 1, wherein a range of a curvatureradius R of the valve body and a curvature radius r of the valve seat isset toR=r=(0.45 to 0.85)Do where Do is a valve seat diameter of the valveseat.
 9. The steam valve according to claim 1, wherein a range of amaximum valve lift La of the valve body combined with the valve seat isset toLa=(0.25 to 0.35)Do where Do is a valve seat diameter of the valve seat.10. The steam valve according to claim 1, wherein a range of a maximumvalve lift Lb of the valve body separated from the valve seat is set toLb<0.25Do where Do is a valve seat diameter of the valve seat.
 11. Thesteam valve according to claim 1, wherein the valve body, the valveseat, and the recessed portion are formed with either one of a hardenedheat-treated portion and a hard-alloy-surfaced portion.
 12. A steamvalve comprising: a valve body; a valve seat; and a valve rod providedfor the valve body, the valve body having a bottom portion to which arecessed portion is formed, the recessed portion having an edge, whereina groove is provided along an entire circumferential surface of thevalve body.
 13. The steam valve according to claim 12, wherein thegroove is formed on a downstream side of a critical point of steamflowing through a steam path defined by the valve body and the valveseat.
 14. A steam valve comprising: a valve body; a valve seat; and avalve rod provided for the valve body, the valve body having a bottomportion to which a recessed portion is formed, the recessed portionhaving an edge, wherein at least one of the valve body and the valveseat is provided with a knurled portion formed on a downstream side of acritical point of steam flowing through a steam path defined by thevalve body and the valve seat.
 15. The steam valve according to claim14, wherein the knurled portion is provided along an entirecircumferential surface of the valve body.